The ongoing development of electronic and optical components has motivated the development of a variety of techniques for thin-film deposition of an ever-broadening array of materials. For semiconductor fabrication, for example, various methods of Chemical Vapor Deposition (CVD) have been developed for dielectric materials, metals and semiconductors. In CVD, a precursor material is delivered in vapor form and mixed with a carrier gas. This process gas mixture is then delivered to a substrate, either by expansion into a well-mixed process volume, or by localized distributed through a showerhead located near the substrate. The process pressures and geometries employed in CVD are generally selected to make use of the viscous flow regime. High substrate temperatures are often maintained in order to facilitate a chemical reaction at the surface, resulting in deposited material onto the heated substrate. Because the CVD apparatus employs viscous flow, the distribution of the process gases by the showerhead and related precursor delivery system conforms to principles of fluid mechanics in the viscous flow regime.
The need for fabrication of organic semiconductor elements and Organic Light Emitting Diode (OLED) displays places even more stringent requirements and restrictions on the vapor deposition process. For example, many organic semiconductor materials are not stable at high temperatures for a long period of time. Depending on their vapor pressure curves these materials may need to be heated significantly in order to entrain sufficient concentration of organic vapor in the carrier gas. Depending on their decomposition characteristics, however, such heating can present the risk of decomposition for such materials. In some cases, maintaining vapor sources at the temperatures required for vaporization for extended periods can cause appreciable decomposition of the organic material over long runs. A method similar to CVD and used for delivering organic vapors to substrates is called the Organic Vapor Phase Deposition (OVPD), as described in U.S. Pat. No. 6,558,736 entitled “Low Pressure Vapor Phase Deposition of Organic Thin Films” to Forrest et al., and has the basic arrangement shown in FIG. 1. A deposition apparatus 10 has a vaporization chamber 12 for vaporizing the organic material. The vaporized material is mixed with a carrier gas 14 and delivered to a showerhead 16 for deposition onto a substrate 18 in a deposition chamber 20.
Improved uniformity is of particular interest for improving thin film vapor deposition. Conventional physical vapor deposition methods (for example, thermal evaporation from small crucibles—so called “point sources”—or elongated crucibles) can achieve thin-film deposition uniform to within about +/−5% of desired thickness. It would be advantageous to improve this level of performance. Some improved methods claim deposition uniformity in the range of about +/−3%. However, there would be particular advantages in providing even better uniformity to within about +/−1%, particularly for devices where the layers are optically active and where thickness variations can cause variations in hue, light output, optical absorption, etc.
Another key area of interest relates to utilization efficiency. Conventional systems generally exhibit poor utilization, often wasting the majority of material. For example, large-area deposition from a limited number of point sources requires large throw distances (that is, source-substrate spacing) to achieve acceptable uniformity. In such cases, as little as 5% of the vaporized material condenses on the substrate, with the majority of the material condensing elsewhere in the deposition chamber. Careful design and set-up of planetary motion substrate holders can make better use of the vaporized material, but this approach is generally limited to batches of a large number of very small substrates, so as to ensure nearly constant radial distance from substrate surface to point source.
In attempting to improve deposition uniformity and utilization for both CVD and OVPD applications, one approach has been to improve the performance of the showerhead. In the case of thermal evaporation (thermal physical vapor deposition), elongated deposition sources have been used with relative motion of the substrate and elongated source perpendicular to the direction of elongation. These elongated sources have been sealed with lid structures having a plurality of apertures, the size and spacing of which can be adjusted to improve uniformity. A few examples of such attempts to improve uniformity and utilization are the following:
U.S. Pat. No. 6,050,506 entitled “Pattern of Apertures in a Showerhead for Chemical Vapor Deposition” to Gui et al. describes a perforation pattern for depositing metals from a CVD showerhead;
U.S. Pat. No. 6,849,241 entitled “Device and Method for Depositing One or More Layers on a Substrate” to Dauelsberg et al. describes an optimized orifice shape for vapor deposition from a showerhead;
U.S. Pat. No. 5,268,034 entitled “Fluid Dispersion Head” and U.S. Pat. No. 5,286,519 entitled “Fluid Dispersion Head for CVD Apparatus”, both to Vukelic, disclose various showerhead perforation patterns for CVD devices;
U.S. Pat. No. 6,565,661 entitled “High Flow Conductance and High Thermal Conductance Showerhead System and Method” to Nguyen describes showerhead designs with variable hole size, pattern, and angular orientation;
U.S. Patent Application Publication No. 2005/0103265 entitled “Gas Distribution Showerhead Featuring Exhaust Apertures” by Gianoulakis et al. describes a showerhead design having both distribution and exhaust apertures in various arrangements;
U.S. Patent Application Publication No. 2005/0087131 entitled “Method and Apparatus for Depositing Material” by Shtein et al. describes forming a pattern on a surface using a collimated, carrier-supported gas jet to perform Organic Vapor Jet Printing;
Commonly assigned U.S. Patent Application Publication No. 2003/0168013 entitled “Elongated Thermal Physical Vapor Deposition Source with Plural Apertures for Making an Organic Light-Emitting Device” by Freeman et al. discloses a linear source for vapor deposition onto a moving substrate in which aperture size and spacing are varied to improve uniformity;
U.S. Patent Application Publication No. 2005/0106319 entitled “Process and Device for Depositing Thin Layers on a Substrate in a Process Chamber of Adjustable Height” by Jurgensen et al. discloses a showerhead used in a chamber that is of adjustable height, for different pressures, for example.
As these examples show, showerhead design is an important factor for achieving uniform deposition of materials for both CVD and OVPD systems, and aperture size and spacing are critical for uniformity in systems based on effusion (Freeman et al). Both CVD and OVPD systems operate in the viscous flow regime, forming a mixture of the vaporized material with an inert or benign carrier gas, then forcing this mixture through showerhead orifices, following the model established with earlier CVD systems. In the fluid dynamics model used for viscous flow, a showerhead is used to direct the vapor/carrier mixture toward the substrate surface. Because this method uses viscous flow, in order to arrive at the substrate surface, the CVD precursor material or OVPD vaporized organic material must then diffuse through a thin boundary layer above the substrate surface.
Another problem with CVD and OVPD systems relates to patterning precision when using shadow masks. Patterned features formed on the substrate surface often exhibit “pillowing” or bowing rather than having sharply defined edges. Referring to the side view of FIG. 2A and top view of FIG. 2B, the ideal shape of a surface feature 22 on substrate 18 would be achieved by having vertical sidewalls 24, parallel to a normal Ns to the surface of substrate 18. However, in practice, as shown in the corresponding side and top views of FIGS. 2C and 2D, side walls 24 are not vertical, but are bowed, typically with a sloped section 26 extending outward, so that an imperfect approximation of straight sidewalls 24 is achieved. As shown in the side view of FIG. 3, vapor underflow causes some portion of material to slip underneath a mask 30, resulting in the sloped section 26 shown in FIG. 2C. This vapor underflow is an undesirable side effect of viscous flow. As a result of this imperfection, increased distances between surface features 22 must be maintained in order to provide distinct features. In component fabrication, this means that device resolution is limited as a result. Surface features 22 with more ideally sloped sidewalls 24, as shown in FIGS. 2A and 2 B, would allow tighter packing of components, reducing the area or “real estate” required for electronic components and thereby increasing the available resolution of a device.
In order to provide higher precision, a number of conventional thermal vapor deposition apparatuses employ a linear source, with orifices arranged along a line. Deposition onto a surface using this type of system requires some type of precision transport mechanism, either to translate the substrate past the showerhead or to translate the showerhead across the substrate surface during deposition. For high coating uniformity, this type of apparatus requires a precision transport mechanism delivering a highly uniform flow of vaporized material.
Yet another drawback of CVD and OVPD apparatus and methods relates to surface temperatures of the substrate. In the classical silicon-based semiconductor model, it was required that the surface temperature of the substrate be very high during deposition, often in excess of 400° C. Hence, CVD systems that were developed for semiconductor fabrication were designed to elevate the temperature of the substrate and to maintain it at high temperatures during materials deposition.
With the advent of organic semiconductors and other organic electronic materials, and with the drive to fabricate devices on organic polymeric substrates, however, substrate temperature concerns are very different from those in the classical semiconductor industry, where the CVD approach has routinely been used to deposit inorganic dielectrics, metals, and semiconductors. Instead of elevated temperatures at the substrate, it is important to maintain the substrate at much lower temperatures when applying organic materials on organic substrates. Furthermore, high-resolution shadow masking (commonly employed to pattern organic electronic devices) requires minimal differential thermal expansion between mask material and substrate, thus further constraining the maximum allowable thermal load. For a vapor deposition process with a significant heat load, it is a requirement to cool the substrate. Deposition processes that operate in the viscous flow regime may cause significant substrate heating, owing to the thermal conductivity of the carrier gas or of the precursor or vaporized material, which transports heat from the exit surface of the heated gas delivery system (showerhead) to the substrate surface.
U.S. Patent Application Publication No. 2004/0255857 entitled “Thin-Film Deposition Evaporator” by Chow et al. discloses a system for depositing a thin-film layer that claims uniformity to within about 3% without any mention of carrier gas use. In order to achieve this level of uniformity, the Chow et al. '5857 disclosure suggests choosing an appropriate arrangement of exit aperture numbers, positions, and dimensions. However, although the Chow et al. '5857 disclosure shows or describes some possible arrangement of manifold apertures, there is no clear indication on how to properly dimension or distribute these apertures to maintain high levels of uniformity. Nor are there any guidelines given for scaling in order to adapt a given deposition system architecture for handling different substrate sizes.
Thus it can be seen that although conventional thermal physical vapor deposition, CVD, and OVPD apparatus and techniques have achieved some measure of success in forming thin-film components and features, there is considerable room for improvement, particularly for fabrication of OLED and related devices.